The thyrotrope-restricted isoform of the retinoid-X receptor-gamma1 mediates 9-cis-retinoic acid suppression of thyrotropin-beta promoter activity

TSHbeta is a subunit of TSH that is uniquely expressed and regulated in the thyrotrope cells of the anterior pituitary gland. Thyroid hormone receptors (TR) are known to mediate T3 suppression of TSHbeta gene expression at the level of promoter activity. The role of other nuclear receptors in regulation of this gene is less clearly defined. Retinoid X receptors (RXR) are a family of nuclear transcription factors that function both as 9-cis-retinoic acid (RA) ligand-dependent receptors and heterodimeric partners with TR and other nuclear receptors. Recently, the RXR isoform, RXRgamma, has been identified in the anterior pituitary gland and found to be restricted to thyrotrope cells within the pitutiary. In this report, we have further characterized the distribution of RXRgamma1, the thyrotrope-restricted isoform of RXRgamma, in murine tissues and different cell types. We have found that RXRgamma1 mRNA and protein are expressed in the TtT-97 thyrotropic tumor, but not the thyrotrope-variant alphaTSH cells or somatotrope-derived GH3 cells. Furthermore, we have studied the effects of RXRgamma1 on TSHbeta promoter activity and hormone regulation in these pituitary-derived cell types. Both T3 and 9-cis-RA independently suppressed promoter activity in the TtT-97 thyrotropes. Interestingly, the combination of ligands suppressed promoter activity more than either alone, indicating that these hormones may act cooperatively to regulate TSHbeta gene expression in thyrotropes. The RXRgamma1 isoform was necessary for the 9-cis-RA-mediated suppression of TSHbeta promoter activity in alphaTSH and GH3 cells, both of which lack this isoform. RXRbeta, a more widely distributed isoform, did not mediate these effects. Finally, we showed that the murine TSHbeta promoter region between -200 and -149 mediated a majority of the 9-cis-RA suppression of promoter activity in thyrotropes. This region is distinct from the T3-mediated response region near the transcription start site. These data suggest that retinoids can mediate TSHbeta gene regulation in thyrotropes and the thyrotrope-restricted isoform, RXRgamma1, is required for this effect.

Thyroid hormone receptor beta2 promoter activity in pituitary cells is regulated by Pit-1

There are three known thyroid hormone receptor (TR) isoforms that arise from two distinct alpha and beta gene loci. TRalpha1 and TRbeta1 mRNAs are found in many tissues, whereas mRNA for the N-terminal TRbeta2 variant derived from the beta locus is readily detectable only in the pituitary gland and derived cell sources such as GH3 somatotropes and TtT-97 thyrotropes. We previously isolated the genomic region governing expression of the TRbeta2 isoform in thyrotropes and showed that transcription arose from multiple origins within a 400-base pair (bp) region. We now report that the region extending 500 bp upstream of the putative AUG codon (A is +1) contains six areas of interaction with the pituitary-specific transcription factor Pit-1. In addition there are separate areas that bind other factors present in thyrotrope cells. Promoter deletions revealed that removal of regions containing the Pit-1 sites at -456 to -432, -149 to -127, and -124 to -102 progressively decreased TRbeta2 promoter activity in thyrotropes. A more proximal footprinted area from -65 to -19, which accounted for the remaining promoter activity, contained sites that interacted with recombinant Pit-1; however, extracts of TtT-97 thyrotropes, which express Pit-1, footprinted this proximal region with a pattern of protection that differed from that produced by Pit-1. A comparative deletional analysis demonstrated that a shorter region extending only 204 bp from the AUG was sufficient to support TRbeta2 promoter activity in GH3 somatotropes. The more proximal Pit-1 sites, including the area from -53 to -19, whose pattern differed from Pit-1 in thyrotrope extracts, showed protection patterns with GH3 extracts that were indistinguishable from recombinant Pit-1. Site-directed mutagenesis that abrogated binding of both recombinant Pit-1 and Pit-1-containing nuclear extracts revealed that the two Pit-1 sites between -149 and -102 were important for TRbeta2 promoter activity with the more proximal being most critical. Finally, we showed that TRbeta2 promoter activity in alpha-TSH cells, which do not transcribe the endogenous TRbeta2 locus or produce Pit-1 protein, could be reconstituted to a level approaching that seen in expressing TtT-97 thyrotropes by cotransfecting a Pit-1 expression vector. Activation by Pit-1 was dependent on the same Pit-1 sites shown to be important for basal TRbeta2 promoter activity in thyrotropes as constructs lacking them by deletion or mutation were not stimulated by Pit-1.

Determinants of thyrotrope-specific thyrotropin beta promoter activation. Cooperation of Pit-1 with another factor

Thyrotropin (TSH) beta is a subunit of TSH, the expression of which is limited to the thyrotrope cells of the anterior pituitary gland. We have utilized the thyrotrope-derived TtT-97 thyrotropic tumors to investigate tissue-specific expression of the TSH beta promoter. TSH beta promoter activity in thyrotropes is conferred by sequences between -270 and -80 of the 5'-flanking region. We have recently reported that the proximal region from -133 to -100 (P1) is required for promoter expression in thyrotropes. This region interacts with the pituitary-specific transcription factor Pit-1. While Pit-1 appears necessary for TSH beta promoter activity in thyrotropes, this transcription factor is not alone sufficient for promoter activity in pituitary-derived cells. In this report, we have generated a series of promoter mutations in the P1 region to identify additional protein-DNA interactions and determine their functional significance. We have found that Pit-1 interacts with the distal portion of the P1 region, and a second protein interacts with the proximal segment of this region. Each protein is able to independently interact with the TSH beta promoter, but neither alone can maintain promoter activity. Both proteins appear to be necessary for full promoter activity in thyrotropes. Southwestern analysis with the proximal segment of the P1 region (-117 to -88) reveals interaction with a 50-kDa protein. Interestingly, this protein is not found in the pituitary-derived GH3 cells and may represent a thyrotrope-specific transcription factor. Further characterization of this newly identified DNA-binding protein will further our understanding of the tissue-specific expression of the TSH beta gene.

N-terminal variants of thyroid hormone receptor beta: differential function and potential contribution to syndrome of resistance to thyroid hormone

The human syndrome of resistance to thyroid hormone (RTH) is associated with dominant mutations in the thyroid hormone receptor beta (TR beta) gene that generate mutant receptors with impaired binding for T3. Although the TR beta gene differentially expresses two N-terminal variant receptors, TR beta 1 and TR beta 2, functional analyses of RTH mutants have focused exclusively on TR beta 1. Since TR beta 2 is expressed in tissues that are malfunctional in RTH, the role of mutations in the context of TR beta 2 was examined. We compared the functional properties of corresponding RTH mutations in the common C-terminal domain of both TR beta 1 and TR beta 2. Wild type TR beta 1 and TR beta 2 bound similarly as homodimers and as heterodimers with retinoid X receptors to T3-responsive elements consisting of a direct repeat with 4-base pair spacing or an everted repeat. Homodimers, but not monomers or heterodimers, of both receptor subtypes were dissociated by the addition of T3. However, TR beta 2 formed at least 10-fold more stable homodimers than TR beta 1 on a palindromic repeat element, indicating that the N termini of TR beta 1 and TR beta 2 differentially influence dimerization on DNA. The RTH-like mutants of both TR beta 1 and TR beta 2 were equally insensitive to T3. They were defective in T3 binding but still bound DNA like their wild type counterparts except that the T3-dependent dissociation of homodimers from DNA was severely reduced. Wild type TR beta 1 and TR beta 2 mediated T3-inducible transactivation in cotransfection assays; this, however, was abolished in both mutants. TR beta 1 mediated more sensitive T3-dependent transcriptional suppression than TR beta 2 through the negative T3 response region of the TSH beta gene. Again, the mutation abolished T3-dependent suppression by both mutants. Furthermore, both mutants inhibited T3-inducible transcriptional activation by different wild type TR alpha and beta variants. These results indicate that both mutants have the potential to contribute to the pathogenesis of RTH and suggest that a reassessment of previous models of RTH is required to take into account the inhibitory activity of both TR beta 2 and TR beta 1 mutants.

Structural and functional characterization of the genomic locus encoding the murine beta 2 thyroid hormone receptor

beta 1 and beta 2 are functional thyroid hormone receptors (TRs) that are generated from the same genomic locus by splicing of a different amino terminus onto a common carboxyl region containing the DNA and hormone binding domains. TR beta 1 is widely expressed whereas TR beta 2 is found primarily in the pituitary gland although low levels of expression have been described in other tissues. To gain insight into the mechanisms governing expression of this complex transcriptional unit, we cloned mouse genomic fragments containing the common carboxyl terminus as well as the unique TR beta 2 amino-terminal sequence that was located at least 25 kilobases upstream. The DNA and ligand binding exons are identical in size and location of their boundaries to those of the human TR beta 1 gene. To determine whether the region 5' of the TR beta 2 amino terminus represented the promoter region, we examined it for sites of transcriptional initiation and for its ability to function as a promoter in TR beta 2-expressing thyrotrope cells. Multiple transcriptional start sites extending over 400 base pairs (bp) were identified with those more proximal showing inhibition by T3. Transcription was not detected more than 400 bp upstream from the putative AUG codon, although initiation downstream of this AUG was demonstrated indicating alternative AUG usage. A fragment containing 500 bp of the TR beta 2 5'-region exhibited preferential promoter activity when transfected into thyrotrope cells that express endogenous TR beta 2. Deletion studies demonstrated that removal of consensus binding sites for the transcription factor Pit-1 resulted in loss of this cell specificity. We therefore conclude that the promoter region responsible for expression of the TR beta 2 isoform in pituitary thyrotropes is distinct from that described for TR beta 1 and is located many kilobases upstream from their common exons.

The combination of Pit-1 and Pit-1T have a synergistic stimulatory effect on the thyrotropin beta-subunit promoter but not the growth hormone or prolactin promoters

Pit-1 is a pituitary-specific transcription factor with protein expression limited to thyrotrope, somatotrope, and lactotrope cells of the anterior pituitary gland. We have recently described a thyrotrope-specific variant isoform of Pit-1, called Pit-1T, which contains an additional 14 amino acids in the activation domain generated by an alternate 3'-splicing choice. Pit-1T, in the presence of Pit-1, stimulates the thyrotropin beta-subunit (TSH beta) promoter in a thyrotrope-derived cell that lacks all Pit-1 isoform proteins. Three laboratories have identified another Pit-1 splice variant, called Pit-1 beta, which contains an additional 26 amino acids in the activation domain that is generated by a similar 3'-alternate splice choice. Pit-1 beta has been shown to stimulate the GH promoter, but not the PRL or TSH beta promoters. In this report, we evaluate the effect of the three Pit-1 isoforms (Pit-1, Pit-1T, and Pit-1 beta) on the GH, PRL, and TSH beta promoters when introduced into different cell types. The combination of Pit-1 and Pit-1T had a synergistic stimulatory effect on the TSH beta promoter, but not on the PRL or GH promoters in a thyrotrope-derived cell line that lacks all Pit-1 protein isoforms (alpha TSH cells). When added to GH3 cells, which lack only the Pit-1T isoform, Pit-1T selectively stimulated the TSH beta promoter and not the GH or PRL promoters, suggesting that the thyrotrope-specific Pit-1T exhibits a promoter-specific effect.(ABSTRACT TRUNCATED AT 250 WORDS)

Secondary malignancy of the thyroid gland: a case report and review of the literature

Metastatic cancer to the thyroid gland is uncommon. In this report we describe a patient with a malignant fibrous histiocytoma that metastasized to the thyroid, possibly to a preexisting thyroid nodule. A review of the literature reveals that breast and lung carcinoma are the most frequently identified sources of secondary thyroid carcinoma found at autopsy, while renal carcinoma comprises over 50% of secondary thyroid malignancies discovered clinically. A number of authors suggest that preexisting thyroid disease (i.e., multinodular goiter and thyroid nodules) may provide a nidus for metastases to the thyroid gland.

A thyrotrope-specific variant of Pit-1 transactivates the thyrotropin beta promoter

Thyrotropin (TSH) beta is a subunit of TSH, the expression of which is limited to the thyrotrope cells of the anterior pituitary gland. Tissue-specific expression of the mouse TSH beta gene is conferred by sequences between -270 and -80 of the 5'-flanking region. We have investigated tissue-specific expression of the TSH beta promoter in two thyrotrope-derived cell types: 1) TtT-97 thyrotropic tumors, which express the endogenous TSH beta gene, and 2) an alpha-TSH cell line, which was generated from a thyrotropic tumor that has lost the ability to express the TSH beta gene. The pituitary-specific transcription factor Pit-1 is present in thyrotropes and interacts with three cis-acting elements in the functionally important region of the TSH beta promoter. Pit-1 protein is present in TtT-97 tumor cells but is absent from alpha-TSH cells. Reintroduction of Pit-1 into alpha-TSH cells by transient transfection does not restore TSH beta promoter activity. We have identified an alternately spliced variant of Pit-1, called Pit-1T, the mRNA and protein expression of which is limited to thyrotrope-derived cells. Pit-1T contains a 14-amino acid insert in the transactivation domain due to an alternate 3' splice acceptor site. Transiently transfected Pit-1T increases TSH beta promoter activity in TtT-97 thyrotropic tumor cells, whereas additional Pit-1 has no effect. The alpha-TSH cell line, which lacks all Pit-1 proteins, requires both isoforms in order to stimulate TSH beta promoter activity. These data suggest that Pit-1T is a thyrotrope-specific splice variant of Pit-1 that is required for TSH beta promoter stimulation; furthermore, both Pit-1 and Pit-1T are required for TSH beta promoter activity in thyrotrope cells.

Analysis of Pit-1 in regulating mouse TSH beta promoter activity in thyrotropes

TSH beta gene expression is restricted to pituitary thyrotropes. Since Pit-1 is present in these cells, we characterized Pit-1 RNA and protein in thyrotropes, and tested its function in regulating TSH beta promoter activity. We demonstrate that both TtT-97 thyrotropic tumors and pituitaries contain four Pit-1 transcripts of 3.2, 2.6, 2.4, and 1.9 kb, respectively. Only two transcripts of 2.7 and 2.1 kb were detected in alpha TSH cells, a thyrotrope derived cell that no longer expresses TSH beta. Western analysis revealed Pit-1 protein in TtT-97 cells but not in alpha TSH cells. DNase I protection assays localized Pit-1 binding to three areas of the mouse TSH beta promoter. However, basal TSH beta promoter activity was minimally stimulated when alpha TSH cells or TtT-97 thyrotropes were co-transfected with mouse Pit-1 and a mTSH beta luciferase construct. These studies suggest that Pit-1 is not limiting for cell-specific expression of the TSH beta gene in thyrotrope-derived cells and implies that additional thyrotropic factors are likely required.